Cheese whey is a byproduct of cheese production. Lactose and protein are the two major components of whey. Lactose recovery from whey has increased steadily over the past years, but lactose remains an underutilized commodity. The value of lactose could be increased if efficient, inexpensive methods existed for converting lactose to higher value chemicals.
Various techniques for converting lactose into other compounds or materials are known. One approach has been to directly hydrogenate lactose to obtain its sugar alcohols, lactitol and lactulitol (see Hu et al., “HPLC and NMR Study of the Reduction of Sweet Whey Permeate”, J. Agric. Food Chem. 44, 3757-3762 (1996)). However, further processing of the sugar alcohol product stream from such a direct hydrogenation produces a wide variety of components ranging from high molecular weight C12 products to lower carbon polyhydric alcohols such as ethylene glycol, propylene glycol and glycerol. Thus, the yield of the lower carbon polyols is less than desirable and significant multiple steps are required for separating the various product components.
A further carbohydrate hydrogenation example is mentioned in U.S. Pat. No. 4,430,253 (“Dubeck”). Dubeck describes a two-stage process for converting carbohydrates to lower carbon polyhydric alcohols. The first stage is a hydrogenation of the carbohydrate to obtain a higher carbon polyhydric alcohol and the second stage is a hydrogenolysis of the higher carbon polyhydric alcohol to obtain the lower carbon polyhydric alcohol. Lactose is included in a list of possible carbohydrates that could undergo conversion according to the process described in Dubeck, but there are no working examples describing conversion of any di- or polysaccharide. Preferred di- or polysaccharide starting materials “are those which are readily hydrolyzable to monosaccharides under dilute acid conditions” (column 3, lines 61-63). Dubeck does not appear to contemplate a continuous process that includes a separate hydrolysis step prior to the hydrogenation. It is stated that “polysaccharides are hydrolyzed to their basic monosaccharide (or monosaccharides) whose aldehyde or ketone groups are then hydrogenated to hydroxyl groups to produce the desired polyhydric alcohol (or alcohols) of the monosaccharide” (column 4, lines 53-57). However, Dubeck teaches that “those polysaccharides having free aldehyde or ketone groups in their molecular structure may have these groups hydrogenated at the same time the molecule is hydrolyzed” (column 4, lines 57-60). The Dubeck disclosure goes on to indicate that a mineral acid “can be added to the first stage reaction medium, either at the outset or during the reaction, e.g., between the first and second stages (the latter is ordinarily preferred), for pH control” (column 6, lines 21-25). Unfortunately, simultaneous hydrogenation and hydrolysis of a disaccharide as suggested by Dubeck tends to reduce product selectivity due to hydrogenation of the disaccharide directly to a C12 alditol and over-reaction via the hydrolysis mechanism to undesired hydrolyzate products, such as hydroxymethylfurfurals. Moreover, the addition of a mineral acid such as sulfuric acid or phosphoric acid can cause contamination of the downstream products and poison the hydrogenation catalyst.
Another example of a lactose manipulation is acid hydrolysis of a partial amount of lactose in a sugar composition in order to sweeten the taste of the composition. However, the currently commercialized processes are terminated prior to achieving hydrolysis of substantially all of the lactose since their objective is not complete conversion of the lactose-containing mixture.
Various techniques for converting glucose to lower carbon polyols are known. For example, U.S. Pat. No. 4,380,678 (“Sirkar”) describes a multi-stage method for converting aldoses to glycerol and other polyols. The Sirkar process involves a first step of catalytically hydrogenating the aldoses to obtain their respective alditols and then subjecting the alditols to catalytic hydrogenolysis to obtain glycerol and other polyols.
In summary, the existing processes for lactose conversion all suffer from significant drawbacks. Thus, a continuing need exists for an efficient, inexpensive method for converting lactose to higher value chemicals.